Brachytherapy is a form of radiotherapy (radiation therapy) used for the treatment of conditions (tumors, for example) of the mammalian body with radiation from within the body rather than external to the body. Because brachytherapy is localized and precise, it is commonly used as an effective radiation treatment for various forms of cancers, such as, but not limited to, cervical, prostate, breast, lung, esophageal, head and neck, and skin cancers. Brachytherapy is typically divided into Low Dose Rate (LDR) brachytherapy and High Dose Rate (HDR) brachytherapy. LDR typically has a dose rate of 40 to 60 cGy/hour (centi-Gray per hour) whereas HDR typically has a dose rate of hundreds (100's) of cGy/minute.
Generally, in HDR brachytherapy systems, one or more thin catheters are first placed in or near the condition within the body and then highly radioactive pellets containing man-made or natural radioactive isotopes are delivered (pushed) into each of the guide catheters. The radioactive sources or seeds are placed in or near the condition, and a high radiation dose (i.e., ionizing radiation, usually X-rays and gamma rays or beta particles from the decay of the isotope) is administered directly to the tumor or tumor bed after a tumor is excised while reducing the radiation exposure in the surrounding healthy tissues and organs. A treatment planning system (TPS) is generally used to determine the optimized treatment for a given condition. This treatment plan is produced to meet the treatment prescribed by the physician. The treatment plan is transferred to or is part of a controller that controls how long the source stays in each dwell position. Each of the plurality of guide catheters may have a number of dwell positions. After a series of treatments, the guide catheters are removed, and there are no radioactive seeds left in the body. Isotope-based brachytherapy systems, however, cannot be turned on and off at will.
Brachytherapy has numerous advantages over other therapies, such as External Beam Radiation Therapy (EBRT), Intensity Modulated Radiation Therapy (IMRT), and Imaged Guided Radiation Therapy (IGRT). For example, in contrast to EBRT, IMRT, and IGRT where high energy X-rays (photons) or electrons are directed at a condition within the body from outside of the body, brachytherapy can treat the condition within the body without delivering radiation through a larger volume of healthy tissue. In contrast to IGRT, in situations where the patient moves or the location of the condition to be treated moves during the treatment (e.g., lung, breast or prostate tumor moving during breathing), the radiation source in brachytherapy retains its correct position in relation to the condition during the treatment, whereas the radiation needs to be guided based on images of the patient or surrogates attached to the patient in IGRT.
EBRT, IMRT and IGRT treatment systems and devices, however, have an advantage over conventional isotope-based brachytherapy systems and devices in that they can be turned on and off at will and there is no residual radioactivity when they are turned off.
Electronic brachytherapy (EBT) is a form of brachytherapy that uses micro miniature accelerators as the radiation source rather than natural or man-made radioisotopes to produce X-rays (photons). Thus, electronic brachytherapy (EBT) has all the advantages of EBRT, IMRT and IGRT, since it can be turned on and off at will, and does not contain any radioactive isotopes. Additionally, just as the isotope-based brachytherapy source, it is contained within the body near or within the condition being treated. Several companies (Xoft, Inc., AXXENT and Carl Zeiss INTRABEAM, for example) have developed EBT systems based on high voltage single stage DC accelerators. The radiation source in these systems is a miniature 50 kVp X-ray source producing low energy X-rays (photons).
Electronic brachytherapy, just like other photon (or X-ray) based therapy treatments, are especially suited for treating cervical, prostate, breast, and lung cancers. Many medical devices have been developed for cervical cancer brachytherapy. An early device was the Fletcher-Suit cervical applicator. The Fletcher applicator provided a method for intracervical or intrauterine guiding of radioactive sources. Derivatives of this device have been developed, for example in Weeks, U.S. Pat. No. 5,562,594; a modified Fletcher tandem device provides shielding for healthy tissue such as the adjacent rectum. Weeks contains a comprehensive dissertation of the physics and anatomy of intracervical brachytherapy, it is incorporated herein by reference. Schoppel et al., U.S. Pat. No. 5,012,357, describes an intracavity brachytherapy applicator that allows inserting and removing shielding for protecting critical structure but allowing computer tomography (CT) imaging without artifacts. Cervical cancer is well treated with radiation but the collateral damage is still of concern. A great deal of effort has gone into shielding critical structures from excess radiation. The need for improved treatment still exists, especially because over 12,000 new cases of cervical cancer are expected to be diagnosed in the US each year with nearly 4,000 deaths.
There are approximately 190,000 new diagnosis of prostate cancer in the US annually with over 32,000 deaths, a significantly lower death rate than cervical cancer. Prostate cancer is well treated with a variety of hormone suppression, radiation and surgical techniques. Low Dose Radiation (LDR) brachytherapy using seed placement was the predominant new treatment over the last decade but advances in IMRT have begun to replace or supplement seeds. Unfortunately, regardless of which approach is used, the effect on quality of life due to collateral damage is still significant.
Lung cancer is the leading cause of cancer deaths in the United States and it claims more lives than colon, prostate and breast cancer combined. Over 196,000 patients are diagnosed with lung cancer each year in the United States and nearly 160,000 die of it. Lung cancer patients are poorly serviced by the current treatment protocols. Cancer tumors in the lung are constantly moving when the patient breathes making precise targeting for IGRT difficult. A new treatment for treating lung cancer is still needed.
In each of the above-described examples, electronic brachytherapy (EBT) provides an advantage. However, photon-based electronic brachytherapy still delivers a significant radiation dose to healthy tissue, which can have serious medical consequences. Moreover, for larger tumors where megavolt (MeV) level treatment is necessary, or where the exponential isotope decay does not produce the desired radiation dose profile, particles, such as electrons, protons, neutrons and heavy ions might be more suitable instead of photons for the treatment.
Current electronic brachytherapy devices, however, do not use particles, such as electrons, protons, neutrons and heavy ions, as the radiation beam source. Instead, X-ray photons are used as the radiation source. Even though, high voltage gradients are used to accelerate particles, typically electrons, these electrons are converted to photons through the Bremsstrahlung radiation process (electrons striking a target) and the photons are used as the treatment radiation, because the electrons are too low in voltage and cannot be used directly in therapy. The Continuously Slowing Down Approximation range of a 50 keV electron is 0.0433 mm (NIST. ESTAR Program). Also, the acceleration potential that is achieved using the currently available high voltage gradients is limited to about 10 MeV/m, which limits the particle voltage to about 100 keV or less in a typical electronic brachytherapy device suitable to be inserted into the body (about 10 mm in length).
In order to use particles such as electrons, protons and heavy ions for brachytherapy treatment, the energy of the particles need to be in the range of at least 5 to 70 MeV. The currently available high voltage gradients are therefore, not suitable for high energy particle generation. The current state of the art superconducting RF cavities produce about 40 MeV/m on a production basis.